Titanium oxide sol, thin film, and processes for producing these

ABSTRACT

An aqueous dispersion of titanium oxide particles comprising chloride ion, and a Brønsted base other than chloride ion, preferably at least one kind of ion selected from nitrate ion and phosphate ion. Preferably the titanium oxide particles are predominantly comprised of brookite titanium oxide particles. The aqueous titanium oxide dispersion is prepared by hydrolyzed titanium tetrachloride in the presence of at least one kind of a Brønsted acid. A thin film formed from this aqueous titanium oxide dispersion exhibits good photo-catalytic activity, transparency and adhesion to a base material.

This application is a 371 of PCT/JP 99/02507 filed May 14, 1999, whichclaims benefit of U.S. Provisional Application No. 60/094,491 filed Jul.29, 1998.

TECHNICAL FIELD

This invention relates to an aqueous dispersion of titanium oxideparticles, a titanium oxide thin film formed on a surface of a basematerial such as ceramic or plastic from the aqueous titanium oxidedispersion, a process for producing the aqueous dispersion of titaniumoxide particles, and a process for producing titanium oxide particlesfrom the aqueous titanium oxide dispersion.

The titanium oxide thin film is transparent, has an excellentphoto-catalytic action and exhibits good adhesion to a base material.

BACKGROUND ART

With respect to titanium dioxide (hereinafter referred to as “titaniumoxide”), it is known that three crystal phases of anatase, brookite andrutile exist. In the case of a vapor phase production process whereintitanium oxide is prepared by the premixed combustion of titaniumtetrachloride and oxygen or the like, anatase titanium oxide is producedat the lowest temperature and this oxide is stable. When the anatasetitanium oxide is heat treated and burnt, brookite titanium oxide isobtained at a temperature in the range from 816° C. to 1,040° C. andrutile titanium oxide is obtained in a temperature range higher thanthis range (see, Rikagaku Jiten (Dictionary of Physicochemistry), 3rded., pp. 514-515).

With respect to a liquid phase production process, Kouemon Funaki has,reported in detail on the crystal phase of titanium oxide produced byhydrolysis of an aqueous titanium tetrachloride solution (see, KogyoKagaku [Industrial Chemistry], Vol. 59, No. 11, p. 1295(1956)). Thisreport states that rutile titanium oxide is produced predominantly froma high concentration solution and anatase titanium oxide is producedpredominantly from a low concentration solution, and further that finelydivided particles of brookite titanium oxide could not be produced in aliquid phase. The starting raw material is titanium tetrachloride andtherefore the resulting titanium oxide inevitably contains chlorine ion.

As seen from these reports, it has heretofore been difficult to stablyprepare brookite titanium oxide by a liquid phase process. When titaniumoxide obtained by a vapor phase process is heat-treated at a hightemperature, the titanium oxide changes into brookite titanium oxide asdescribed above, however, the particles grow due to the heat treatment,and accordingly, it has heretofore been difficult to obtain finelydivided titanium oxide particles with brookite crystal form.

On the other hand, a sol, i.e., an aqueous dispersion of titanium oxideparticles is generally produced by dispersing crystalline or amorphoustitanium oxide particles in a dispersion medium, or incorporating aprecursor of titanium oxide such as a titanium alkoxide, titaniumsulfate or titanium tetrachloride in a dispersion medium, and thenneutralizing or hydrolyzing the dispersion or mixture to form a sol.

The aqueous titanium oxide dispersion is used for producing a titaniumoxide powder or forming a titanium oxide thin film on a surface ofglass, plastic or other materials by coating the aqueous dispersion ontothe surface.

The titanium oxide is a photo-semiconductor and known to exhibittransparency and improved photo-catalytic function when the particlesize is small. The photo-catalytic function of titanium oxide is beingaggressively investigated and studied in recent years. Thisphoto-catalyst is used for stain-proofing by removing harmfulsubstances, deodorization of malodorous gas such as ammonia, orsterilization of microbes, and according to the purpose of use, thetitanium oxide is formed into various shapes such as bulk particles,thin film and a sol. In the case of obtaining transparency in additionto the photo-catalytic function, the titanium oxide is most often formedinto a thin film. To this purpose, the titanium oxide as a film formingmaterial is used in the form of a sol, i.e., an aqueous dispersion.

As for the photo-catalytic capacity of titanium oxide, it is known thatthe anatase type surpasses the rutile type. This is ascribable to thedifference in the energy gap between the two types. The rutile type hasan energy gap of 3.02 eV and the anatase type has 3.23 eV, thus, thedifference between the two types of crystal forms is about 0.2 eV (see,Ceramics 31, No. 10, p. 817 (1996). Due to this difference in energygap, anatase titanium oxide having a high energy gap is conventionallyused as a photo-semiconductor. However, heretofore no case is knownwhere brookite titanium oxide is extracted as a single substance.Moreover, it has been difficult to produce finely divided brookitetitanium oxide particles having a high specific surface area and capableof use as a photo-semiconductor (photo-catalyst) because the particlesare undesirably sintered due to the production process employing a hightemperature. Thus, the capacity of the brookite titanium oxide as aphoto-catalyst is quite unknown.

In recent years, there has been proposed a process of coating a sol offinely divided titanium oxide particles on a lighting equipment such asglass tube or cover of a fluorescent lamp to form a thin film, and usingthe thin film for decomposing by the photo-catalytic action thereof anorganic material such as lamp black adhering to the glass tube or cover,thereby preventing pollution of the glass tube or cover. However, when athin file is formed from the aqueous titanium oxide dispersion obtainedby above-described process, a thin film having high transparency isdifficult to obtain. In particular, use of a brookite titanium oxidethin film as a photo-catalyst for lighting equipments or other articlesis heretofore not known.

In recent years, there has proposed a process of coating a sol of finelydivided titanium oxide particles on a lighting equipment such as glasstube or cover of a fluorecent lamp to form a thin film, and using thethin film for decomposing by the photo-catalytic action thereof anorganic material such as lamp black adhering to the glass tube or cover,thereby preventing pollution of the glass tube or cover. However, when athin film is formed the aqueous titanium oxide dispersion obtained byabove-described process, a thin film having high transparency isdifficult to obtain. In particular, use of a brookite titanium oxidethin film as a photo-catalyst for lighting equipments or other articlesis heretofore not known.

In the case of forming a titanium oxide thin film on a base materialmade of glass, plastic or other substances and using the thin film as aphoto-catalyst, the thin film is required to have a high photo-catalyticactivity. The photo-catalytic action is a reaction occurring on thesurface of a particle and, in order to attain a high photo-catalyticactivity, the particle is preferably a finely divided particle having ahigh specific surface area. When a thin film is formed on lightingequipments or other articles, the thin film must be transparent andthus, similarly to the photo-catalytic activity, finely dividedparticles are preferable so as to attain good transparency, moreover, adispersion of primary particles is preferred. Conventionally, theserequirements have been dealt with mainly by using finely divided anatasetitanium oxide particles.

In the case of forming a titanium oxide thin film on a base material,good adhesion must be attained between the thin film and the basematerial, otherwise, the thin film is readily stripped off.

According to the conventional production process comprising hydrolyzingtitanium tetrachloride, it has been very difficult to produce an aqueoustitanium oxide dispersion comprising finely divided particles having avery small particle size, a high crystallinity and, when formed into athin film, exhibits good transparency.

The titanium oxide in a sol produced by the hydrolysis of a titaniumalkoxide compound may have good powder properties such that the particlesize is very small, however, the titanium alkoxide compound is veryexpensive as compared with titanium tetrachloride.

DISCLOSURE OF THE INVENTION

In view of the foregoing prior art, an object of the present inventionis to provide a sol, i.e. an aqueous dispersion of finely dividedtitanium oxide particles characterized in that, when the sol is coatedon a surface of a base material of various types to form a titaniumoxide thin film on the base material surface, the thin film exhibitsgood photo-catalytic function, high transparency and sufficiently highadhesion between the thin film and the base material. Another object isto provide a thin film formed from the aqueous titanium oxide dispersionA further object is to provide a process for producing finely dividedtitanium oxide particles from the aqueous titanium oxide dispersion.

As a result of extensive investigations on a titanium oxide thin filmformed from an aqueous titanium oxide dispersion, the present inventorshave found that, by allowing a Brønsted base other than chlorine ion,preferably either one or both of nitrate ion and phosphate ion to existtogether with chloride ion in the aqueous titanium oxide dispersion goodthin film properties can be obtained, for example, the transparency isgood and the adhesion between the base material and the thin film issufficiently high, and further that the titanium oxide predominantlycomprised of brookite titanium oxide has a photo-catalytic capacityequal to or higher than the photo-catalytic capacity of anatase titaniumoxide. The present invention has been accomplished based on thesefinding.

Thus, in one aspect of the present invention, there is provided anaqueous dispersion of finely divided titanium oxide particles comprisingchloride ion and at least one kind of Brønsted base selected from thegroup consisting of pyrophosphate ion, metaphosphate ion, polyphosphateion, methanesulfonate ion, ethanesulfonate ion, dodecylbenzenesulfonateion and propanesulfonate ion. The term “Brønsted base” herein used, wemean a proton acceptor in Brønsted acid-base concept.

In another aspect of the present invention, there is provided a titaniumoxide thin film formed on a surface of a base material from theabove-mentioned aqueous dispersion of finely divided titanium oxideparticles.

In still another aspect of the present invention, there is provided anarticle made by coating a surface of a base material with theabove-mentioned aqueous dispersion of finely divided titanium oxideparticles.

In a further aspect of the present invention, there is provided aprocess for producing an aqueous dispersion of finely divided titaniumoxide particles, especially a dispersion of finely divided titaniumoxide particles predominantly comprised of finely divided brookitetitanium oxide particles, which dispersion comprises chloride ion and aBrønsted base other than chloride ion, characterized in that titaniumtetrachloride is hydrolyzed in the presence of at least one kind ofBrønsted acid selected from the group consisting of nitrate ion,phosphate ion, pyrophosphate ion, metaphosphate ion, polyphosphate ion,methanesulfonate ion, ethanesulfonate ion, dodecylbenzene-sulfonate ionand propanesulfonate ion; and a process for producing finely dividedtitanium oxide particles characterized by obtaining the titanium oxideparticles from the aqueous titanium oxide dispersion prepared by theabove-mentioned process.

In a further aspect of the present invention, there is provided aprocess for producing an aqueous dispersion of finely divided titaniumoxide particles predominantly comprised of finely divided brookitetitanium oxide particles, which dispersion comprises chloride ion andeither one or both of nitrate ion and phosphate ion, characterized inthat titanium tetrachloride is incorporated in hot water maintained at atemperature of 75° C. to 100° C., and then, the titanium tetrachlorideis hydrolyzed in the presence of either one or both of nitrate ion andphosphate ion at a temperature in the range of 75° C. to the boilingpoint of an aqueous reaction solution; and a process for producingtitanium oxide particles, characterized by obtaining finely dividedtitanium oxide particles predominantly comprised of finely dividedbrookite titanium oxide particles, from the aqueous dispersion ofbrookite titanium oxide particles prepared by the above-mentionedprocess.

BRIEF DESCRIPTION OF THE DRAWING

FIG. 1 is a schematic cross-sectional view showing one preferableexample of a reaction vessel used for the production of an aqueoustitanium oxide dispersion of the present invention.

BEST MODE FOR CARRYING OUT THE INVENTION

The aqueous titanium oxide dispersion of the present invention compriseschloride ion and at least one Brønsted base other than chloride ion. Athin film formed from the aqueous titanium oxide dispersion has not onlyexcellent photo-catalytic function but also high adhesion to the basematerial and improved transparency.

The Brønsted base contained in the aqueous titanium oxide dispersion ofthe present invention is preferably selected from nitrate ion, phosphateion, pyrophosphate ion, metaphosphate ion, polyphosphate ion, acetateion and organic acid ions. These Brønsted bases may be contained eitheralone or as a combination of at least two thereof. As specific examplesof the organic acid ions, there can be mentioned methanesulfonic acid,ethanesulfonic acid, dodecylbenzenesulfonic acid and propanesulfonicacid. The amount of the Brønsted base contained in the aqueous titaniumoxide dispersion of the present invention means not the amount of theBrønsted base which is present in equilibrium state in the aqueousdispersion, but the absolute amount of the total ions which are presentin the aqueous dispersion. The content of chloride ion and the Brønstedbase other than chloride ion is preferably in the range of 50 ppm to10,000 ppm, more preferably 100 ppm to 4,000 ppm, as the total anioncontent in the titanium oxide sol.

The dispersion medium of the aqueous titanium oxide dispersion isusually water or a mixture of water and an organic solvent. In the caseof preparing the aqueous titanium oxide dispersion by hydrolyzingtitanium tetrachloride, hydrogen chloride is generated during thehydrolysis reaction, and dissociated into chloride ion and hydrogen ionin the resulting aqueous dispersion. This hydrogen chloride generallyescapes from the system in many cases during the hydrolysis reactionunder heating. It is considered that the presence of hydrogen chloridein the aqueous dispersion incurs various troubles when a titanium oxidepowder is prepared or a titanium oxide thin film is formed from theaqueous dispersion. Accordingly, if a certain amount of hydrogenchloride remains in the sol after completion of the hydrolysis reaction,the aqueous dispersion is usually subjected to a dechlorinationtreatment to reduce the hydrogen chloride content in the aqueousdispersion as much as possible. However, effects of this chloride ion inthe aqueous dispersion on the properties of the titanium oxide thin filmhave heretofore been not studied and no attempt has been made forcontrolling the chloride ion in the aqueous dispersion from this aspect.

The present inventors have previously found that when chloride ion iscontained in the aqueous dispersion, a titanium oxide thin film formedfrom the aqueous dispersion has a high photo-catalytic activity andexcellent adhesion to a base material. Thereafter, studies have beencontinued to have found that when chloride ion, and at least oneBrønsted base other than chloride ion, such as nitrate ion or phosphateion, are present together, the thin film formed exhibits more improvedtransparency and adhesion. The present invention has been accomplishedbased on these findings. The reason for which the improved results areobtained is not clear, but it is presumed that a condensation reactionoccurs due to the acid catalyst effect of a co-existing Brønsted acidsuch as nitric acid or phosphoric acid, and the film-forming propertyand adhesion of the thin film are enhanced.

It may be sufficient that chloride ion and at least one kind of an ionselected from Brønsted bases other than chloride ion (said ion ishereinafter referred to as “chloride ion and the like” when appropriate)are present together in the aqueous titanium oxide dispersion, however,in order to increase the adhesion between the titanium oxide thin filmformed on a base material and the base material, chloride ion and thelike are preferably contained in the total amount of at least 50 ppm. Inparticular, in the case where the thin film is calcined, the adhesion isimproved when chloride ion and the like are contained in the totalamount of at least 50 ppm. If the total amount of chloride ion and thelike increases in the aqueous dispersion and exceeds 10,000 ppm, thetransparency of the thin film is deteriorated. The total amount isparticularly preferably in the range of from 100 ppm to 4,000 ppm.

The ratio of Brønsted base other than chloride ion to chloride ion isnot particularly limited, and the total amount of chloride ion andBrønsted base other than chloride ion may be selected over a wide range,for example, from 0.1 mol to 200 mols per mol of chloride ion.

It is surprising that brookite titanium oxide can be produced bymaintaining the total amount of chloride ion and Brønsted base otherthan chloride ion in the above-mentioned range. Although the reason isnot clear, the aqueous titanium oxide dispersion containing brookitecrystal exhibits high photo-catalytic activity and results in a thinfilm having good transparency as compared with an aqueous titanium oxidedispersion containing anatase crystal or rutile crystal alone or anaqueous titanium oxide dispersion containing both of anatase crystal andrutile crystal. The content of brookite crystal in the aqueousdispersion is not particularly limited but is preferably in the range of10% to 100% by weight, and more preferably 50% to 100% by weight.

The action of chloride ion and the like is not clearly elucidated but itis presumed that electrical repulsion among titanium oxide particles inthe aqueous titanium oxide dispersion increases to bring about gooddispersion of particles, as a result, the above-described good effectson the transparency and peel strength can be obtained.

As the titanium oxide particles in the aqueous titanium oxide dispersionare smaller, the photo-catalytic activity and transparency of thetitanium oxide thin film are more improved. However, excessively smalltitanium oxide particles are difficult to produce. Accordingly, thetitanium particles in the aqueous dispersion preferably have an averageprimary particle diameter in the range of 0.01 μm to 0.1 μm.

In order to further increase the photo-catalytic function andtransparency of the thin film formed from the aqueous titanium oxidedispersion is preferably an aqueous dispersion such that titanium oxideparticles predominantly comprised of brookite titanium oxide particlesand having an average particle diameter in the range of from 0.01 μm to0.1 μm and a specific surface area of at least 20 m²/g are dispersed inwater or a mixture of water and an organic solvent.

As a process for producing brookite titanium oxide particles, heretoforeknown is only a process of heat-treating anatase titanium oxideparticles as described above. If formation of a thin film is attemptedusing the brookite titanium oxide particles obtained by the heattreatment, a thin film cannot be successfully formed because theparticle size is inevitably greatly increased due to sintering duringthe heat treatment. Accordingly, this brookite titanium oxide particleshave not been used at all for forming a thin film.

In the aqueous titanium oxide dispersion of the present invention, ifthe concentration of titanium oxide particles is too high, the particlescoagulates to render the aqueous dispersion unstable. On the other hand,if the concentration of titanium oxide particles is too low, there ariseproblems, for example, the process of coating the aqueous dispersiontakes a long period of time for the thin film formation. Accordingly,the concentration (content) of titanium oxide particles in the aqueoustitanium oxide dispersion is suitably in the range of from 0.05mol/liter to 10 mol/liter.

The aqueous titanium oxide dispersion of the present invention may beprocessed in a usual manner, for example, through filtering, waterwashing and drying, to obtain titanium oxide particles. The titaniumoxide particles preferably have an average primary particle diameter inthe range of from 0.01 μm to 0.1 μm. The titanium oxide particlesobtained from an aqueous titanium oxide dispersion predominantlycomprised of brookite titanium oxide particles preferably have anaverage particle in the range of from 0.01 μm to 0.1 μm and a specificsurface area of at least 20 m²/g.

In the case of using the aqueous titanium oxide dispersion for forming athin film, a small amount, for example, from about 10 ppm to about10,000 ppm of a water-soluble polymer may be added so as to increase thethin film forming property of the aqueous dispersion. Suitable examplesof the water-soluble polymer include polyvinyl alcohol, methylcellulose, ethyl cellulose, CMC and starch.

The aqueous titanium oxide dispersion of the present invention may becoated on a base material to form a titanium oxide thin film on the basematerial surface. As the base material, various materials and shapedarticles may be used and, for example, ceramics, metals, plastics, wood,paper and the like may be used almost without any limitation. Also, thebase material may be previously coated. Furthermore, the base materialmay comprise alumina, zirconia or the like which can work out to acatalyst support, and after allowing the titanium oxide thin filmcatalyst to be supported thereon, the base material may be used as acatalyst. When glass, plastic cover or the like of lighting equipmentssuch as fluorescent lamp is used as a base material and a titanium oxidethin film is formed thereon, the thin film is transparent and has aphoto-catalytic activity and accordingly, the thin film can decompose anorganic material such as lamp black without shielding the light andthus, is effective for preventing pollution of the glass or cover. Whena titanium oxide thin film is formed on an architectural glass or wallmaterial, the thin film can similarly prevent pollution. Accordingly,the titanium oxide thin film may be formed on the window material orwall material of a tall building and this can dispense with cleaningoperation, in turn, the cost for managing the building can be curtailed.

The aqueous titanium oxide dispersion may be coated on a base materialby a process of dipping the base material in the aqueous dispersion, aprocess of spraying the aqueous dispersion on the base material, or aprocess of coating the base material with the aqueous dispersion byusing a brush. The amount of the aqueous dispersion applied for coatingis preferably from 0.01 mm to 0.2 mm expressed in terms of thickness ofthe liquid coating. After the coating, the base material coated with theaqueous dispersion is dried to remove the moisture content and thus, athin film is obtained. This thin film can be used as it is as a catalystor for others.

In the case where the base material is made of heat-resistant substance,such as metal, ceramic or glass, the titanium oxide thin film formed maybe calcined. By the calcination, the thin film is more tightly bonded tothe base material and the hardness of the thin film is enhanced. Thecalcination temperature is preferably at least 200° C. The upper limitof the calcination temperature is not particularly limited and may beselected according to the degree of heat resistance of the basematerial. However, even if an excessively high temperature is employed,hardness of the thin film and adhesion to the base material do notincrease any more. Accordingly, the calcination temperature is suitablyup to about 800° C. In the case of titanium oxide particles mainlycomprised of brookite titanium oxide particles, the calcination ispreferably performed at a temperature of 700° C. or lower so as tomaintain the brookite crystal form. Especially preferably, an aqueoustitanium oxide dispersion comprising chloride ion and phosphate ion,which has been prepared by hydrolyzing titanium tetrachloride in thepresence of phosphate ion, can be formed into a thin film having goodadhesion and high hardness by calcining the aqueous dispersion at arelatively low temperature, i.e., at least 200° C. but lower than 500°C. without incorporation of an adhesive therein.

The calcination atmosphere is not particularly limited and thecalcination can be performed in an air atmosphere. The calcination timeis not particularly limited and, for example, from 1 to 60 minutes. Thethickness of the titanium oxide thin film with the above-describedcoated amount is approximately in the range of from about 0.05 μm toabout 1.0 μm.

In order to firmly bond the transparent thin film of the presentinvention to the base material and further increase the adhesionstrength thereof, an appropriate adhesive may be added to the aqueoustitanium oxide dispersion. As an example of the adhesive, there can bementioned an organic silica compound such as an alkyl silicate. Theamount of the adhesive added is not particularly limited, however, inthe case of alkyl silicate, the added amount thereof is, in terms ofSiO₂, approximately from 1% to 50% by weight based on the titanium oxidein the aqueous dispersion of the present invention. If the amount of theadhesive added is smaller than 1% by weight, the effect by the additionis small. In contrast, if it exceeds 50% by weight, the adhesivestrength to the base material may be very intensified but the titaniumoxide particle is thoroughly coated with the adhesive and thephoto-catalytic function is diminished. The adhesive may be mixedimmediately before the film formation or may be previously mixed intothe aqueous titanium oxide dispersion. Either process may be selecteddepending on the property of the adhesive. Whichever is selected, theeffect of the present invention is not adversely affected. The thin filmcontaining the adhesive may or may not be calcined.

The titanium oxide thin film prepared using the aqueous titanium oxidedispersion of the present invention is crystalline, comprises veryfinely divided titanium oxide particles which are free of impurities,and allows the finely divided titanium oxide particles to disperseexceedingly alike primary particles. Accordingly, the thin film has highphoto-catalytic capability and high transparency. In particular, whenthe titanium oxide is predominantly comprised of, i.e., comprises atleast 50% by weight of brookite titanium oxide, the photo-catalyticactivity is more increased.

The process for producing the aqueous titanium oxide dispersion of thepresent invention will now be described.

It is sufficient that the aqueous titanium oxide dispersion of thepresent invention contains chloride ion and the Brønsted base other thanchloride ion preferably in the above-described amount, and thepreparation process thereof is not particularly limited. For example, aprocess of hydrolyzing a titanium alkoxide compound to obtain an aqueousdispersion of titanium oxide particles containing a small amount of analcohol, adding hydrogen chloride or other chlorides thereto, andfurther adding thereto either one or both of nitrate ion and phosphateion to have a concentration of the chloride ion plus nitrate and/orphosphate ion preferably falling within the above-described range may beemployed. However, titanium tetrachloride which involves the generationof hydrogen chloride during the hydrolysis is preferably used. Theprocess for producing an aqueous dispersion of titanium oxide particleswill be described below by referring to the case where titaniumtetrachloride is used.

The hydrogen chloride generated during the hydrolysis of titaniumtetrachloride is preferably prevented from escaping out of the reactiontank and allowed to remain in the aqueous dispersion as much aspossible. If the titanium tetrachloride is hydrolyzed while allowing thehydrogen chloride generated to escape, the titanium oxide in the aqueousdispersion encounters difficulties in attaining a small particle sizeand further, its crystallinity is deteriorated.

The hydrogen chloride generated during the hydrolysis may not becompletely prevented from escaping but it is sufficient if the escapingis suppressed. The process therefor is also not particularly limited asfar as the escaping can be suppressed. For example, a pressure may beapplied thereto but a most simple and effective process is to performthe hydrolysis in a hydrolysis reaction vessel equipped with a refluxcondenser. FIG. 1 shows this apparatus. In the figure, a reaction vessel1 filled with an aqueous solution 2 of titanium tetrachloride isequipped with a reflux condenser 3, a stirrer 4, a thermometer 5 and adevice 6 for heating the reaction vessel. During the hydrolysisreaction, water vapor and hydrogen chloride vapor are generated and mostof the vapors are condensed through the reflux condenser 3 and returnedto the reaction vessel 1. Accordingly, the hydrogen chloride scarcelyescapes outside from the reaction vessel 1.

If the concentration of titanium tetrachloride in the aqueous titaniumtetrachloride solution hydrolyzed is too low, productivity is poor andefficiency in the formation of a thin film from the aqueous titaniumoxide dispersion produced is low. On the other hand, if theconcentration is excessively high, the reaction vigorously proceeds,therefore, finely divided titanium oxide particles are difficult toobtain, and the dispersibility is poor and as such, the aqueousdispersion is disadvantageous as a transparent film-forming material.The titanium tetrachloride concentration is preferably in the range offrom 0.05mol/liter to 10 mol/liter. By hydrolyzing this aqueous titaniumtetrachloride solution underheating, a sol, i.e., an aqueous dispersioncomprising titanium oxide (TiO₂) particles dispersed therein isobtained. When the hydrolysis is performed using a reaction vesselequipped with a reflux condenser, the aqueous dispersion obtained hastitanium oxide concentration of approximately from 0.05 mol/liter to 10mol/liter and accordingly, the thus produced aqueous dispersion can beused as it is as a coating material having a preferred titanium oxideconcentration. If desired, water may be added to the aqueous dispersionas obtained by the hydrolysis or the aqueous dispersion may beconcentrated, so that the titanium oxide concentration can fall withinthe above-described preferred range.

The hydrolysis temperature is preferably from50° C. to the boiling pointof the aqueous titanium tetrachloride solution. If the hydrolysistemperature is lower than 50° C., the hydrolysis reaction takes a longperiod of time. The hydrolysis is performed by elevating the temperatureto a temperature within the above-described range and maintaining thetemperature for approximately from 10 minutes to 12 hours. The time formaintaining the temperature may be shorter as the hydrolysis temperatureis in the higher side.

The aqueous titanium tetrachloride solution may be hydrolyzed by heatinga solution of titanium tetrachloride in water in a reaction vessel at apredetermined temperature, or by previously heating water in a reactionvessel, adding titanium tetrachloride therein, and then, heating themixed solution at a predetermined temperature. The titanium oxideobtained by this hydrolysis is generally a mixture comprised of apredominant proportion of brookite titanium oxide with minor amounts ofanatase titanium oxide and/or rutile titanium oxide. For obtainingtitanium oxide containing a higher amount of brookite titanium oxide, aprocess of previously heating water in a reaction vessel at atemperature of from 75° C. to 100° C., preferably from 85° C. to 95° C.,adding thereto titanium tetrachloride, and then, performing thehydrolysis at a temperature of from 75° C. to the boiling point of thesolution, or, when the previous heating temperature is in the range offrom 85° C. to 95° C., performing the hydrolysis at a temperature offrom 85° C. to the boiling point of the solution is preferred.

By this process, titanium oxide containing at least 70% by weight ofbrookite titanium oxide based on the total weight of titanium oxide canbe prepared. The reason for which such a high proportion of brookitetitanium oxide is produced is not clearly elucidated, but it is to benoted that the titanium oxide containing a salient proportion ofbrookite titanium oxide is obtained from a mixture comprising chlorideion and Brønsted acid other than chloride ion by performing thehydrolysis at a temperature of from 50° C. to the boiling point of theaqueous titanium tetrachloride solution. Preferable examples of theBrønsted acid are nitric acid and phosphoric acid, which have a boilingpoint higher than that of hydrochloric acid.

As the temperature of the aqueous titanium tetrachloride solution to behydrolyzed is elevated at a higher rate, the particles obtained can befiner. Accordingly, the temperature elevating rate is preferably atleast 0.2° C./min, more preferably at least 0.5° C./min. By thisprocess, titanium oxide particles in the aqueous dispersion can have apreferred average particle diameter in the range of from 0.01 μm to 0.1μm and exhibit good crystallinity.

The type of preparation of the aqueous titanium oxide dispersion of thepresent invention is not limited, and a batch system may be employed,and a continuous system may also be employed wherein a continuousreaction vessel is used where titanium tetrachloride and water arecontinuously charged therein through an inlet, and the reaction solutionis taken out from an outlet located opposite to the inlet andsubsequently subjected to a dechlorination treatment.

The thus-prepared aqueous dispersion is adjusted to have a chloride ionconcentration of lower than 10,000 ppm by a dechlorination treatment orby addition of water or dehydration within the range of causing notrouble.

The dechlorination treatment may be performed by generally known meansand for example, electrodialysis, ion exchange resin or electrolysis maybe used. To the aqueous dispersion adjusted to have a chloride ionconcentration of lower than 10,000 ppm, at least one Brønsted base,preferably either one or both of nitric acid and phosphoric acid areadded and the aqueous dispersion is adjusted to have a total amount ofthese ions of from 50 ppm to 10,000 ppm. The nitric acid, phosphoricacid and other Brønsted base may also be added at the time of hydrolysisof titanium tetrachloride.

The dispersion medium of the aqueous titanium oxide dispersion of thepresent invention is generally water or a mixture of water and anorganic solvent. The organic solvent is added to the aqueous dispersionafter the hydrolysis of titanium tetrachloride, or may be added to theaqueous titanium tetrachloride solution and the resulting solution maybe hydrolyzed. Also in the case of the dispersion medium containing anorganic solvent, the titanium oxide concentration in the aqueousdispersion as a coating material is preferably in the range of from 0.05mol/liter to 10 mol/liter. In the case of adding an organic solventafter the preparation of the aqueous dispersion, the aqueous dispersionmay first be concentrated or dehydrated, if desired, to increase thetitanium oxide concentration and then an organic solvent may be addedthereto, so that the titanium oxide concentration in the aqueousdispersion can be adjusted to fall within the above-described range.

The organic solvent is preferably hydrophilic, and specific examplesthereof include monohydric or polyhydric alcohols such as methanol,ethanol and ethylene glycol, ketones such as acetone, esters such asethyl acetate, and CELLOSOLVES such as ethyl CELLOSOLVE. These may beused either alone or as a mixture. The organic solvent may be added tothe aqueous dispersion in any amount but preferably added in an amountof not larger than 2,000 parts by weight per 100 parts by weight ofwater in the aqueous dispersion.

In the case where a titanium oxide thin film is formed from the aqueoustitanium oxide dispersion of the present invention, the aqueousdispersion as produced by the hydrolysis reaction is preferably used asit is. A process of firstly producing a titanium oxide powder from theas-produced aqueous dispersion and then dispersing the powder in waterto form an aqueous titanium oxide dispersion for use is not preferred.This is because the titanium oxide particle has a high surface activityand as the particle size becomes finer and finer, the activity moreincreases and thus dispersion of finely divided particles into waterbecomes very difficult, that is, agglomerates are produced. The thinfilm formed from this sol has poor transparency and reducedphoto-catalytic action.

The present invention will now be described in detail by referring tothe following working examples, but the present invention is by no meanslimited to these examples.

In the working examples, the identification of the crystal structure oftitanium oxide and the content of the titanium oxide having a crystalstructure identified were determined as follows.

The X-ray peaks of three main crystal systems of titanium oxide, namely,brookite, anatase and rutile, are overlapped in the major part as seenin Table 1 (extract from JCPDS Card). In particular, the d values in themain peaks (intensity ratio: 100) of brookite and anatase crystals are3.51 (crystal face: 120) and 3.52 (crystal face: 101), respectively, and2θ by the Cu tube bulb in the X-ray diffraction is in the vicinity of25.4°. The angle difference by 2θ is 0.1° or smaller and thus, the peaksare overlapped. Accordingly, the contents of two types of crystalscannot be determined from the intensity ratio of the main peaks thereof.The brookite also has a d value at 3.47 (crystal face: 111). 2θ in thesethree peaks is from 25.4° to 25.7° and thus, the peaks are substantiallyoverlapped.

As such, the intensity ratio of the main peaks between the brookite andthe anatase cannot be obtained. In the working examples, the peak of the121 face of the brookite, which is not overlapped with the peak of theanatase, was used and an intensity ratio of this peak to the peak wherethe above-described three peaks are overlapped (peak intensity of 121face of brookite)/(peak intensity where three peaks are overlapped) wasobtained. From the intensity ratio obtained, the contents of brookitetitanium oxide and anatase titanium oxide were determined. The contentof the rutile titanium oxide was determined from an intensity ratio ofthe main peak (110 face) of the rutile type titanium oxide to the peakwhere three peaks are overlapped (main peak intensity of rutile)/(peakintensity where three peaks are overlapped). In actual measurement,identification by the X ray diffraction using an X ray diffractionapparatus (RAD-B Rotor Flex, supplied by Rigaku Denki KK) andquantitative analysis by a data processing were performed together.

TABLE 1 Extract of JCPDS Card (Card No.) Brookite (29-1360) Anatase(21-1272) Rutile (21-1276) d Crystal Intensity d Crystal Intensity dCrystal Intensity Value face ratio Value face ratio Value face ratio3.51 120 100  3.52 101 100  3.25 110 100  2.90 121 90 1.89 200 35 1.69211 60 3.47 111 80 2.38 004 20 2.49 101 50

In the working examples, the following physical properties of a thinfilm were evaluated as characteristics of an aqueous titanium oxidedispersion.

Evaluation of Properties of Thin Film

2 ml of the coating solution prepared in each of Examples andComparative Examples was coated on a soda lime glass having a size of 76mm×26 mm. The glass was kept perpendicularly for 10 minutes to removethe excess coating solution. After the completion of coating, the glasswith the coating solution was dried and calcined at a predeterminedtemperature described in each of Examples and Comparative Examples(hereinafter referred to as “film-forming temperature” which means acalcination temperature) to obtain a titanium oxide thin film (filmthickness: about 0.2 μm). Transparency, photo-catalytic activity, tightadhesion, adhesive strength and pencil hardness of the thin film formedwas evaluated. The results obtained are shown in Table 3.

Transparency

The transparency was determined according to JIS K6718 using a hazemeter, manufactured by Tokyo Denshoku Gijutsu Center KK, and evaluatedaccording to the following three ratings.

A: Haze ratio of smaller than 2.0%

B: Haze ratio of at least 2.0% but smaller than 5.0%

C: Haze ratio of at least 5%

Photo-catalytic Activity

The Photo-catalytic activity was determined by coating a few drops ofred ink on the base material, irradiating the coating with black light(365 nm) at an ultraviolet intensity of 2.1 mW/cm² for 30 minutes, andobserving the fading of the red ink by the naked eye. The results wereexpressed according to the following three ratings.

A: Well discolored

B: Partially not discolored

C: Not discolored

Tight Adhesion

The tight adhesion to the base material was evaluated by a water wipingtest and an alcohol wiping test. The soda lime glass base material wasrubbed in 10 reciprocation motions with KIMWIPE™ (produced by Crecia)wetted with water or an alcohol and then wiped off with dry KIMWIPE™ in10 reciprocation motions. Thereafter, the film state was evaluated bythe naked eye and expressed by the following three ratings.

A: The film was not scratched.

B. The film was partially scratched.

C. The film was partially stripped off.

Adhesive Strength

The adhesive strength was determined by a cross-cut tape adhesion testaccording to JIS K5400, where the scratch interval was 1 mm and thenumber of squares was 100. The adhesive strength was expressed by thenumber of squares among 100 squares, which were not peeled.

Pencil Hardness

The pencil hardness was determined according to the pencil hardness testmethod (JIS K5400).

EXAMPLE 1

To titanium tetrachloride (purity: 99.9%), water was added to prepare anaqueous titanium tetrachloride solution having a concentration of 0.25mol/liter (2% by weight as titanium oxide). At this time, the system wasice cooled so that the liquid temperature of the aqueous solution didnot exceed 50° C. Thereafter, 1 liter of this aqueous solution wascharged into a reaction vessel with a reflux condenser as shown in FIG.1 and heated to a temperature (104° C.) in the vicinity of the boilingpoint. The mixed solution was kept at the same temperature for 60 hoursto effect hydrolysis. The thus-obtained aqueous dispersion was cooledand the residual chlorine produced by the reaction was removed byelectrodialysis to have a chloride ion (C1 ion) concentration of 1,000ppm. The electrodialysis was performed by an electrodialyser Model G3manufactured by Asahi Chemical Industry Co., Ltd. while monitoring thepH of the aqueous dispersion. Then, nitric acid was added to the aqueousdispersion to have a NO₃ ion concentration of 3,000 ppm.

To the aqueous titanium oxide dispersion adjusted to have theabove-described chloride ion and nitrate ion concentrations, awater-soluble polymer polyvinyl alcohol as a thin film formation aid wasadded in an amount of 1,000 ppm based on the weight of the aqueousdispersion. Even after one or more days, precipitation of finely dividedtitanium oxide particles produced was not observed in the aqueousdispersion.

A part of particles in the aqueous dispersion were sampled, andobservation of the particles through a transmission type electronmicroscope revealed that the average particle diameter was 0.018 μm.Thereafter, the crystal structure of the titanium oxide was examined byX ray diffraction. As a result, the X ray peak intensity ratio (peakintensity of 121 face of brookite/peak intensity where three peaks areoverlapped) was 0.35 and there was not observed a peak of rutile type.The titanium oxide obtained was crystalline, and from the peak intensityratio determined, it was proved to consist of about 40% by weight ofanatase titanium oxide and about 60% by weight of brookite titaniumoxide.

Then, an ethanol solution of tetraethyl-o-silicate (concentration: 0.25%by weight as SiO₂) as an adhesive was prepared.

The aqueous dispersion obtained above was mixed together with thisadhesive solution at a weight ratio of 1:1 to prepare a coating materialof the aqueous titanium oxide dispersion. This coating material had acomposition shown in Table 2. According the above-described method, thinfilm was formed from the coating material and its properties wereevaluated wherein the heating temperature of the coating was 100° C. Theevaluation results are shown in Table 3.

EXAMPLE 2

By the same procedures as described in Example 1, an aqueous titaniumoxide dispersion was prepared and thin film was formed from the aqueousdispersion and evaluated. The composition of the coating material andthe process for preparation thereof were identical to those employed inExample 1, but the heating (calcination) temperature was varied to 400°C. The composition of the coating material is shown in Table 2 and theevaluation results of the thin film are shown in Table 3.

Comparative Examples 1 to 4

By the same procedures as described in Example 1, an aqueous titaniumoxide dispersion was prepared and thin film was formed from the aqueousdispersion and evaluated. Nitric acid was not added and the amount ofchloride ion was varied as shown in Table 3. Tetraethyl ortho-silicateas an adhesive was not added in Comparative Examples 1 and 2. Thecomposition of the coating materials are shown in Table 2 and theevaluation results of the thin films are shown in Table 3.

EXAMPLES 3 to 12

By the same procedures as described in Example 1, aqueous titanium oxidedispersions were prepared, and thin films were formed therefrom andproperties thereof were evaluated. The amounts of phosphate ion andnitrate ion were varied. The compositions of the coating materials areshown in Table 2, and the evaluation results are shown in Table 3.

Particle sizes and crystal forms of titanium oxide particles obtained inComparative Examples 1 and 2 and Examples 3 to 12 were almost the sameas those of Examples 1 and 2.

EXAMPLE 13

954 ml of distilled water was charged in a reaction vessel with a refluxcondenser as shown in FIG. 1 and heated at 95° C. Then, phosphoric acidwas added to have a concentration as PO₄ of 200 ppm. To this aqueoussolution in the reaction vessel, 46 ml of an aqueous titaniumtetrachloride solution (Ti content: 16.3% by weight, specific gravity:1.59, purity: 99.9% by weight) was added dropwise at a rate of about 5ml/min while keeping the stirring rate at about 200 rpm. At this time,the dropwise addition was carefully made so as not to lower thetemperature of the reaction solution. As a result, the titaniumtetrachloride concentration was 0.25 mol/liter (2% by weight as titaniumoxide).

In the reaction vessel, the reaction solution started to become turbidwhite immediately after the dropwise addition but the temperature waskept as it was, and after the completion of dropwise addition, thetemperature was further elevated near to the boiling point (104° C.).The reaction solution was held in this state for 60 minutes to completethe reaction. After the cooling, the residual chlorine produced by thereaction was removed by electrodialysis to adjust the pH to 1.9(chloride ion: 600 ppm, phosphate ion: 200 ppm). Thereafter, awater-soluble polymer polyvinyl alcohol as a thin film formation aid wasadded in an amount of 0.1% by weight based on the titanium oxide contentto prepare an aqueous dispersion of titanium oxide (TiO₂:about 0.25mol/liter). This aqueous dispersion was stable and eve n after 30 ormore days, precipitation of finely divided titanium oxide particlesproduced was not observed.

A part of the aqueous titanium oxide dispersion obtained above wasfiltered, formed into a powder by a vacuum drier at 60° C. and takenout, and the powder was subjected to quantitative analysis in the samemanner as in the previous working examples. As a result, the ratio of(peak intensity of 121 face of brookite/peak intensity where three peaksare overlapped) was 0.38 and the ratio of (main peak intensity ofrutile/peak intensity where three peaks are overlapped) was 0.05. Fromthese, it was determined that the titanium oxide was crystalline, andconsisted of about 70.0% by weight of brookite, about 1.2% by weight ofrutile and about 28.8% by weight of anatase. Observation of the finelydivided particles through a transmission type electron microscoperevealed that the primary particles had an average particle diameter of0.015 μm. Further, the finely divided particles had a specific surfacearea of 140 m^(2/)g as determined by the BET method.

A thin film was made and properties thereof were evaluated in the samemanner as in the previous examples. The composition of the coating,material is shown in Table 2, and the evaluation results are shown inTable 3.

EXAMPLE 14

Hydrolysis of titanium tetrachloride was performed under the samehydrolysis conditions as in Example 1 to obtain an aqueous dispersioncontaining 0.25 mol/liter of finely divided titanium oxide particles.Before performing electrodialysis, the aqueous dispersion wasconcentrated by evaporation to have a titanium oxide concentration of2.5 mol/liter (20% by weight as titanium oxide). Thereafter, theresidual chlorine was removed by electrodialysis to have a chloride ionconcentration of about 1,200 ppm.

Subsequently, nitric acid and phosphoric acid were added to the aqueousdispersion to have a concentration of 2,000 ppm as NO₃ ion and 1,000 ppmas PO₄ ion, respectively. To the resulting aqueous titanium oxidedispersion adjusted to contain nitrate ion and phosphate ion asdescribed above, methyl alcohol as a solvent and a water-soluble polymerpolyvinyl alcohol were added in amounts shown in Table 2, therebyobtaining an aqueous titanium oxide dispersion (TiO₂:about 0.5mol/liter).

Using this aqueous dispersion, a thin film was formed in the same manneras above and properties of the thin film were evaluated. The evaluationresults are shown in Table 3.

The average particle diameter, the ratio among crystal forms and thespecific surface area of titanium oxide particles were almost the sameas those in Examples 1 and 2.

TABLE 2 Composition of Coating Material Examples and Ethanol*₁ orPolyvinyl Tetraethyl- Chloride Nitrate Phosphate Titanium ComparativeWater Methanol*₂ alcohol o-silicate ion (Cl) ion (NO₃) ion (PO₄) oxide(TiO₂) Examples (wt %) (wt %) (wt %) (wt % as SiO₂) (wt ppm) (wt ppm)(wt ppm) (mol/l) Example 1 49 49*₁ 0.1 0.125  500 1500 — 0.125 Example 249 49*₁ 0.1 0.125  500 1500 — 0.125 Example 3 49 49*₁ 0.1 0.125 1000 —200 0.125 Example 4 49 49*₁ 0.1 0.125 1000 — 200 0.125 Example 5 49 49*₁0.1 0.125  100  50 — 0.125 Example 6 49 49*₁ 0.1 0.125  100  50 — 0.125Example 7 49 49*₁ 0.1 0.125 2000 — 100 0.125 Example 8 49 49*₁ 0.1 0.1252000 — 100 0.125 Example 9 49 49*₁ 0.1 0.125 1000 1000 100 0.125 Example10 49 49*₁ 0.1 0.125 1000 1000 100 0.125 Example 11 49 49*₁ 0.1 0.1251500 1000 500 0.125 Example 12 49 49*₁ 0.1 0.125 1500 1000 500 0.125Example 13 98 — 0.1 —  600 — 200 0.25  Example 14 49 49*₂ 0.1 —  6001000 600 0.125 Com. Ex. 1 49 49*₁ 0.1 — 1000 — — 0.125 Com. Ex. 2 4949*₁ 0.1 — 1000 — — 0.125 Com. Ex. 3 49 49*₁ 0.1 0.125 1000 — — 0.125Com. Ex. 4 49 49*₁ 0.1 0.125 1000 — — 0.125

TABLE 3 Evaluation of Thin Film Examples and Film-forming Photo- TightTight Adhesive Comparative temperature catalytic adhesion, adhesion,strength Pencil Examples (° C.) Transparency activity water*1 alcohol*2*3 hardness Example 1 100 A A A B 90 4H Example 2 400 A B A A 90 4HExample 3 100 B A A B 90 4H Example 4 400 B A A A 90 4H Example 5 100 AA A B 90 4H Example 6 400 A B A A 90 4H Example 7 100 B A A B 95 4HExample 8 400 B A A A 95 5H Example 9 100 A A A B 95 4H Example 10 400 AA A A 95 5H Example 11 100 A A A B 95 4H Example 12 400 A A A A 95 5HExample 13 400 A A A A 100  7H Example 14 400 A A A A 100  7H Com. Ex. 1100 B A C C 80 HB Com. Ex. 2 400 B B B C 85 2H Com. Ex. 3 100 B A B C 852H Com. Ex. 4 400 B B B C 85 2H *1Water wiping test, *2Alcohol wipingtest, *3Number of squares not stripped/100 squares

INDUSTRIAL APPLICABILITY

The aqueous dispersion of titanium oxide particles of the presentinvention comprises chloride ion and at least one kind of Brønsted baseother than chloride ion. When the aqueous dispersion of titanium oxideof the present invention is coated on a base material of various typesto form a titanium oxide thin film, the thin film is transparent andexhibits excellent photo-catalytic activity. Especially, in the case ofbrookite titanium oxide, a strong photo-catalytic activity is obtained.Furthermore, the thin film has high hardness and exhibits excellentadhesion to the base material.

Accordingly, the thin film on the base material has good durability and,when this thin film is formed on a glass tube or cover of lightingequipment, the photocatalytic activity can be maintained over a longperiod of time without shielding the light.

The aqueous dispersion of titanium oxide particles of the presentinvention can be prepared in an aqueous system starting from titaniumtetrachloride and is advantageous in that the starting material isinexpensive, the aqueous dispersion can be easily formed into a thinfilm in an economically advantageous manner.

A thin film formed from an aqueous brookite titanium oxide dispersioncontaining nitrate ion has an especially high transparency. An aqueoustitanium oxide dispersion containing phosphate ion is advantageous inthat, when a thin film formed therefrom on a soda glass base material iscalcined, deterioration of photocatalytic activity due to calcinationcan be prevented or minimized, and thus, a high photocatalytic activitycan be obtained.

What is claimed is:
 1. An aqueous dispersion of titanium oxide particlescomprising chloride ion and at least one Brønsted base selected from thegroup consisting of pyrophosphate ion, metaphosphate ion, polyphosphateion, methanesulfonate ion, ethanesulfonate ion, dodecylbenzenesulfonateion and propanesulfonate ion, wherein the titanium oxide particles havean average particle diameter of from about 0.01 μm to about 0.1 μm. 2.The aqueous titanium oxide dispersion as claimed in claim 1, wherein thecontent of chloride ion and the Brønsted base is in the range of about50 ppm to about 10,000 ppm as the total anion content in the aqueoustitanium oxide dispersion.
 3. The aqueous titanium oxide dispersion asclaimed in claim 1, wherein the content of titanium oxide particles inthe aqueous titanium oxide dispersion is in the range of about 0.05mol/liter to about 10 mol/liter.
 4. The aqueous titanium oxidedispersion as claimed in claim 1, which further contains from about 10ppm to about 10,000 ppm of a water-soluble polymer.
 5. The aqueousdispersion of titanium oxide particles as claimed in claim 1, whereinthe Brønsted base is selected from the group consisting of metaphosphateion, methanesulfonate ion, ethanesulfonate ion, dodecylbenzenesulfonateion and propanesulfonate ion.
 6. The aqueous titanium oxide dispersionas claimed in claim 1, which further contains an adhesive.
 7. Theaqueous titanium oxide dispersion as claimed in claim 6, wherein theadhesive is an alkyl silicate.
 8. An article made by coating a surfaceof a base material with the aqueous dispersion of titanium oxideparticles as claimed in claim
 1. 9. The article as claimed in claim 8,wherein the base material is made of at least one substance selectedfrom the group consisting of ceramics, metals, glass, plastics, paperand wood.
 10. A titanium oxide thin film formed on a surface of a basematerial with the aqueous dispersion of titanium oxide particles asclaimed in claim
 1. 11. The titanium oxide thin film as claimed in claim10, wherein the base material is made of at least one substance selectedfrom the group consisting of ceramics, metals, glass, plastics, paperand wood.
 12. The titanium oxide thin film as claimed in claim 10,wherein the base material is made of at least one heat-resistantsubstance selected from the group consisting of ceramics, metals and,glass, and the titanium oxide thin film has been calcined.
 13. Anaqueous titanium oxide dispersion comprising chloride ion and a Brønstedbase selected from the group consisting of nitrate ion, phosphate ion,pyrophosphate ion, metaphosphate ion, polyphosphate ion and an organicacid ion, which is a dispersion of titanium oxide particles comprisingat least 70% by weight of brookite titanium oxide particles having anaverage particle diameter of from about 0.01 μm to about 0.1 μm and aspecific surface area of at least about 20 m²/g.
 14. The aqueoustitanium oxide dispersion as claimed in claim 13, wherein Brønsted baseis at least one ion selected from the group consisting of nitrate ionand phosphate ion.
 15. The aqueous titanium oxide dispersion as claimedin claim 13, wherein the content of titanium oxide particles in theaqueous titanium oxide dispersion is in the range of about 0.05mol/liter to about 10 mol/liter.
 16. The aqueous titanium oxidedispersion as claimed in claim 13, which further contains from about 10ppm to about 10,000 ppm of a water-soluble polymer.
 17. The aqueoustitanium oxide dispersion as claimed in claim 13, which further containsan adhesive.
 18. An article made by coating a surface of a base materialwith the aqueous dispersion of titanium oxide particles as claimed inclaim
 13. 19. A titanium oxide thin film formed on a surface of a basematerial with the aqueous dispersion of titanium oxide particles asclaimed in claim
 13. 20. The titanium oxide thin film as claimed inclaim 19, wherein the base material is made of at least one substanceselected from the group consisting of ceramics, metals, glass, plastics,paper and wood.
 21. An aqueous dispersion of titanium oxide particlescontaining chloride ion and a Brønsted base selected from the groupconsisting of nitrate ion, phosphate ions, pyrophosphate ion,metaphosphate ion, polyphosphate ion and an organic acid ion, andfurther containing a binder comprising an alkyl silicate.
 22. A processfor producing an aqueous dispersion of titanium oxide particlescomprising chloride ion and at least one Brønsted base other thanchloride ion, characterized in that titanium tetrachloride is hydrolyzedin the presence of a solution containing at least one Brønsted base fromthe group consisting of nitrate ion, phosphate ion, pyrophosphate ion,metaphosphate ion, polyphosphate ion, methanesulfonate ion,ethanesulfonate ion, dodecylbenzenesulfonate ion and propanesulfonateion.
 23. The process for producing the aqueous titanium oxide dispersionas claimed in claim 22, wherein the content of chloride ion and theBrønsted base is made to fall in the range of about 50 ppm to about10,000 ppm as the total anion content in the aqueous titanium oxidedispersion.
 24. The process for producing the aqueous dispersion oftitanium oxide particles as claimed in claim 22, wherein the hydrolysisof titanium tetrachloride is carried out in a reaction vessel equippedwith a reflux condenser.
 25. A process for producing titanium oxideparticles characterized by obtaining the titanium oxide particles fromthe aqueous titanium oxide dispersion prepared by the process as claimedin claim
 22. 26. A process for producing an aqueous dispersion oftitanium oxide particles, characterized in that titanium tetrachlorideis hydrolyzed in the presence of a solution comprising a Brønsted base,which is at least one ion selected from the group consisting of nitrateion and phosphate ion; and the content of chloride ion and the Brønstedbase is made to fall in the range of about 50 ppm to about 10,000 ppm asthe total anion content in the aqueous titanium oxide dispersion.
 27. Aprocess for producing titanium oxide particles characterized byobtaining the titanium oxide particles from the aqueous titanium oxidedispersion prepared by the process as claimed in claim
 26. 28. A processfor producing an aqueous dispersion of titanium oxide particlescomprising brookite titanium oxide particles, which comprises chlorideion and a Brønsted base other than chloride ion, characterized in thattitanium tetrachloride is hydrolyzed in the presence of a solutioncontaining at least one Brønsted base selected from the group consistingof nitrate ion, phosphate ion, pyrophosphate ion, metaphosphate ion,polyphosphate ion, methanesulfonate ion, ethanesulfonate ion,dodecylbenzenesulfonate ion and propanesulfonate ion.
 29. The processfor producing the aqueous brookite titanium oxide dispersion as claimedin claim 28, wherein the content of chloride ion and the Brønsted baseother than chloride ion is made to fall in the range of about 50 ppm toabout 10,000 ppm as the total anion content in the aqueous brookitetitanium oxide dispersion.
 30. The process for producing the aqueousdispersion of titanium oxide particles as claimed in claim 29, whereinthe hydrolysis of titanium tetrachloride is carried out in a reactionvessel equipped with a reflux condenser.
 31. A process for producingfinely divided brookite titanium oxide particles, characterized byobtaining the titanium oxide particles from the aqueous brookitetitanium oxide dispersion prepared by the process as claimed in claim29.
 32. A process for producing an aqueous dispersion of titanium oxideparticles predominantly comprised of brookite titanium oxide particles,which comprises chloride ion and at least one ion selected from thegroup consisting of nitrate ion and phosphate ion, characterized in thattitanium tetrachloride is hydrolyzed in the presence of at least one ionselected from the group consisting of nitrate ion and phosphate ion at atemperature in the range from about 75° C. to the boiling point of anaqueous reaction solution.
 33. The process for producing the aqueousdispersion of titanium oxide particles Predominantly comprised ofbrookite titanium oxide particles, as claimed in claim 32 wherein thecontent of chloride ion and at least one kind of ion selected from thegroup consisting of nitrate ion and phosphate ion is made to fall in therange of about 50 ppm to about 10,000 ppm as the total anion content inthe aqueous brookite titanium oxide dispersion.
 34. The process forproducing the aqueous dispersion of titanium oxide particles as claimedin claim 32, wherein the hydrolysis of titanium tetrachloride is carriedout in a reaction vessel equipped with a reflux condenser.
 35. A processfor producing titanium oxide particles, characterized by obtainingtitanium oxide particles predominantly comprised of brookite titaniumoxide particles, from the aqueous titanium oxide dispersion prepared bythe process as claimed in claim 32.